Devices having a balloon mounted at the distal end of a catheter are useful in a variety of medical procedures. A balloon reservoir may be used to deliver a biologically compatible fluid, such a radiologically opaque fluid for contrast x-rays, to a site within the body. Radial expansion of a balloon may be used to expand or inflate a stent positioned within the body. A balloon may also be used to widen a vessel into which the catheter is inserted by dilating the blocked vessel. For example, in the technique of balloon angioplasty, a catheter is inserted for long distances into blood vessels of extremely reduced diameter and used to release or dilate stenoses therein by balloon inflation. These applications require extremely thin walled high strength relatively inelastic balloons of accurately predictable inflation properties.
Dilatation balloons made from PET (polyethylene terephthalate) are well known and widely used for angioplasty, stent placement, treatments in the gastrointestinal, urethral, or reproductive tracts, and for other medical purposes. Other polymer materials have also been reported to be useful in such applications and some of those polymer materials have also been used commercially, for instance, polyethylene, polyvinyl chloride, Surlyn(copyright) polyethylene ionomer copolymer, nylon 12 and Pebax(copyright) polyamide-polyether-polyester block copolymer. In a number of references pertaining to the formation of dilatation balloons, PBT (polybutylene terephthalate) is mentioned as a suitable balloon material, alone or as one layer of a laminate balloon. Such statements may be found in EP 0 745 395 A2 (Ethicon); U.S. Pat. No. 5,270,086 (Hamlin); and U.S. Pat. No. 5,304,340 (Downey). To date, however, no reference has actually reported the preparation of a PBT dilatation balloon, or even a PBT balloon layer.
Balloons made from poly(butylene terephthalate)-block-poly(tetramethylene oxide) are described in U.S. Pat. No. 5,556,383 (L. Wang, et al), incorporated herein by reference. In U.S. Pat. No. 5,316,016 there is described a diagnostic imaging balloon for use on catheters to obtain an image of the configuration of an internal lesion or other body structure. The balloon has a memory effect when inflated with a pressure in a certain low pressure range. The polymer material used to prepare the balloon is a blend of PBT and a non-crystallizing ethylene/cyclohexanedimethylene terephthalate copolyester. This document states that it may be possible to prepare such balloons using PBT alone but to date no such balloons have been prepared. The imaging balloons prepared according to this patent are not suitable for dilatation or for other high pressure applications.
A major reason why PBT has not been used to make such balloons is the extremely high crystallization rate which the polymer displays. The high crystallization rate of polybutylene terephthalate makes it especially suitable as a molding resin where rapid crystallization reduces mold residence time. However for articles which are prepared from an extruded parison by a blow molding process, use of PBT polymer material has proven to be extremely difficult or impossible. This is because even very rapidly quenched extrusions are typically so high in crystallinity that the parson cannot be further processed in a practical manner. Opacification of the parison, another effect of PBT""s high crystallinity, may also have been perceived as a problem for quality control in a manufacturing process. References describing the crystallization rate of PBT and/or its impact on thermoforming applications include: M. Gilbert, et al., xe2x80x9cEffect of Chemical Structure on Crystallization Rates and Melting of Polymers: Part 1. Aromatic Polyesters,xe2x80x9d Polymer. 13, 327-332 (7/72); E. Chang, et al., xe2x80x9cThe Effect of Additives on the Crystallization of Poly(Butylene Terephthalate).xe2x80x9dPolymer Engineering and Science, 18, 932-936 (9/78); U.S. Pat. No. 5,213,754 Kawaguchi, et al. (5/93); and U.S. Pat. No. 5,128,404, Howe (7/92).
In U.S. Pat. No. 5,213,754 there is described a polyester container prepared from a melt molded film of a butylene terephthalate copolyester. The copolyester is prepared from terephthalic acid, 1,4-butane diol and an alkylene oxide adduct of a bisphenol compound. The copolyester is used to provide a lower crystallization rate compared to PBT homopolymer material. The lower crystallization rate taken together with specific subsequent processing steps allows a thermoformable sheet to be obtained.
In U.S. Pat. No. 5,128,404 blow moldable PBT blend compositions are prepared by melt blending a mixture consisting essentially of PBT, an ethylene copolymer containing epoxide groups and an ionomer obtained by neutralizing a (meth)acrylic acid functional polymer with Na+ or K+.
In U.S. Pat. No. 4,380,621 crystallization rate of alkaline carboxylate terminated PBT is reported to have been increased by use of boric acid as a polymer additive or of sodium borate as a polymerization additive.
It would be desirable to be able to make medical balloons such as dilatation or stent placement balloons out of PBT because the material offers the potential of achieving strength properties similar to PET but with better rewrap and lesion crossing characteristics. There therefore exists a need for improved processing techniques or for improved formulations which allow formation of high strength PBT balloons.
The present invention relates, in one aspect, to a processing technique which allows compositions of PBT polymers or copolymers to be formed into blow molded articles, especially medical balloons such as dilatation or stent placement balloons, from extruded tubular parisons. The process includes a longitudinal necking step run at a temperature at or below the Tg of the polymeric material and a radial expansion step run at a temperature above the Tg of the polymeric material. A characterizing feature of the inventive process is that, during the longitudinal necking step, the tubing is subjected to high internal pressure.
In another aspect the present invention relates to novel PBT-containing formulations which provide improved blow molding processability while producing a high strength formed article. According to this aspect of the invention, it has been discovered that a small amount of boric acid added to the polybutylene terephthalate in a melt blend gives a formulation which has reduced crystallinity after extrusion, as evidenced by improved extrusion clarity, and which can more readily fashioned into high strength blow molded articles than PBT by itself.
While the processing and formulation aspects of the invention can be practiced independently of each other, in preferred embodiments of the invention they are practiced together.
The invention is also directed to articles, especially medical device balloon articles, formed by blow molding of a polymer composition in which the polymer consists essentially of PBT. Articles formed using the process of the invention and/or the PBT/boric acid formulation described herein are also within the scope of the present invention.